(…) The thermal and epithermal self-shielding factors, Gth and Gep[equivalente a Gres], are very close to unity for most activation reactions in most samples and can be ignored. However, the self-shielding effect is important for elements with high thermal neutron absorption cross-sections or high resonance neutron activation cross sections if these elements are present in high concentrations and for large samples. For most elements, epithermal neutron self-shielding is more significant than thermal neutron self-shielding. Historically, the calculation of Gth and Gep was extremely difficult, and it was recommended to dilute the samples to avoid self-shielding. Fortunately,reactor physicists recently showed that the amount of epithermal as well as thermal self-shielding could be expressed by the same analytical function, a sigmoid, for all nuclides. [1–3](…)